- IMAGE POSTERIZATION -

Posterization occurs when an image's apparent
bit depth has been decreased so much
that it has a visual impact. The term posterization is used because
it can influence your photo similar to how the colors may look in a mass-produced
poster, where the print process uses a limited number of color inks. This
effect ranges from subtle to quite pronounced, although one's tolerance for
posterization may vary.

Any process which "stretches" the
histogram has the potential to cause posterization. Stretching can
be caused by techniques such as
levels and
curves in Photoshop, or by converting an image from one
color space into another as part of
color
management. The best way to ward off posterization is to keep any
histogram manipulation to a minimum.

Visually inspecting an image is a good way to detect posterization, however
the best objective tool is the histogram. Although RGB histograms will
show extreme cases, the individual
color histograms are your most sensitive means of diagnosis. The two
RGB histograms below demonstrate an extreme case, where a previously narrow
histogram has been stretched to almost three times its original width.

Note the tell-tale sign of posterization on the right: vertical spikes which
look similar to the teeth of a comb. Why does it look like this?
Recall that each channel in an 8-bit image can only have discrete color intensities
from 0 to 255 (see "Understanding Bit
Depth"). A stretched histogram is forced to spread these discrete
levels over a broader range than exists in the original image. This creates
gaps where there is no longer any intensity information left in the image.
As an example, if we were to take a color histogram which ranged from 120 to
130 and then stretched it from 100 to 150 (5x its original width), then there
would be peaks at every increment of 5 (100, 105, 110, etc) and no pixels in
between. Visually, this would force colors to "jump" or form steps in
what would otherwise be smooth color gradations. Keep in mind though that
all digital images have discrete color levels—it is only when these levels sufficiently
disperse that our eye is able to perceive them.

Posterization occurs more easily in regions of gradual color transitions,
such as smooth skies. These regions require more colors levels to describe
them and so any decrease in levels can have a visual impact on the image.

USEFUL TIPS

Using images with 16-bits per channel can greatly reduce the risk of
posterization since this provides up to 256 times as many color levels as
8-bit. Realistically, you can expect anywhere from 4-16 times as many
levels if your image originated from a digital camera since most capture
at 10 to 12-bits per channel in RAW mode— regardless of whether or not you
saved it as a 16-bit file.

Adjustment layers in Photoshop will decrease the likelihood of unnecessarily
performing the same image manipulations more than once.

Even if your original image was 8-bits per channel, performing all editing
in 16-bit mode will nearly eliminate posterization caused by rounding errors.

Working in color spaces with broad gamuts can increase the likelihood
of posterization because they require more bit depth to produce the same
color gradient.